gh-160561: Close the duplicated descriptor when fromfd() is rejected - #160579

socket.fromfd() duplicates the descriptor and then hands it to the constructor. If the constructor rejects its arguments, the duplicate has no owner and is never closed.

fd = sock.fileno()
for _ in range(3):
    try:
        socket.fromfd(fd, socket.AF_INET, socket.SOCK_STREAM, 1 << 100)
    except OverflowError:
        pass

That leaks 3 descriptors; with the change, 0. A valid call still returns a working socket and the descriptor count is unchanged.

The test counts /proc/self/fd before and after one rejected call, so it is skipped where /proc is absent. It passes on this branch and fails against unmodified Lib/socket.py with AssertionError: 7 != 6.

./python -m unittest test.test_socket.BasicTCPTest.testFromFdInvalidArgsDoesNotLeak is OK, and test_socket is green on the free-threading debug build (run=755, skipped=50).

NEWS entry included. No behaviour change on the success path.

• Issue: socket.fromfd() leaks the dup()'d fd when family/type/proto are invalid #160561